1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to an In-Plane Switching (IPS) mode LCD device and its fabrication method.
2. Background of the Related Art
An LCD device is commonly used for producing high quality images while using low power consumption. The LCD device includes a thin film transistor (TFT) array substrate and a color filter substrate attached together in an opposing manner with a uniform interval therebetween, and a liquid crystal layer is formed between the TFT array substrate and the color filter substrate. The TFT array substrate includes pixel units arranged in a matrix configuration, and each unit pixel includes a TFT, a pixel electrode, and a capacitor. The color filter substrate includes a common electrode for inducing an electric field together with the pixel electrode to the liquid crystal layer, an RGB color filter for achieving colors, and a black matrix.
An alignment film is formed on opposing surfaces of the TFT array substrate and the color filter substrate, and a rubbing technique is performed to initially align the liquid crystal layer along a certain direction. When an electric field is generated between the pixel electrode formed according to each unit pixel on the TFT array substrate and the common electrode formed on the entire surface of the color filter substrate, liquid crystals of the liquid crystal layer rotate due to dielectric anisotropy, thereby transmitting or blocking light of each of the unit pixels in order to display a character or an image. However, the above-described twisted nematic (TN) mode LCD device is disadvantageous since its viewing angle is narrow. Thus, an In-Plane Switching (IPS) mode LCD is being developed that solves the problem of narrow viewing angle by aligning liquid crystal molecules along an almost horizontal (in-plane) direction.
FIG. 1A is a plan view of an In-Plane Switching mode LCD device according to the related art, and FIG. 1B is a cross-sectional view along I-I′ of FIG. 1A an In-Plane Switching mode LCD device according to the related art. In FIG. 1A, gate lines I and data lines 3 are arranged vertically and horizontally to define a pixel region on a transparent first substrate 10. Specifically, an n-number of the gate lines 1 and an m-number of the data lines 3 cross over to make an n-by-m number of pixels. However, for purposes of brevity, only a single representative unit pixel is shown in FIGS. 1A and 1B.
In FIG. 1A, a TFT 9 including a gate electrode 1a, a semiconductor layer 5, and source/drain electrodes 2a and 2b are disposed at the crossing of the gate line 1 and the data line 3. The gate electrode 1a and the source/drain electrodes 2a and 2b are connected with the gate line 1 and the data line 3, respectively, and a gate insulation film 8 (in FIG. 1B) is stacked over the entire surface of the substrate.
In the pixel region defined by the gate and data lines 1 and 3, a common line 16 is arranged to be parallel with the gate line 1, and a pair of electrodes (i.e., a common electrode 6 and a pixel electrode 7 for switching liquid crystal molecules) are arranged in parallel with the data line 3. The common electrode 6 is simultaneously formed together with the gate line 1 and is connected with the common line 16, and the pixel electrode 7 is connected with the drain electrode 2b of the TFT 9. Next, a passivation film 11 (in FIG. 1B) is formed entirely on the substrate 10 including the source/drain electrodes 2a and 2b, and the pixel electrode 7 is formed on the passivation film 11. Then, a pixel electrode line 17 overlapping with the common line 16 and connected with the pixel electrode 7 forms a storage capacitor Cst with the gate insulation film 8 and the passivation film 11 interposed therebetween.
In FIG. 1B, a black matrix 21 for preventing light leakage to the TFT 9, the gate line 1, and the data line 3, and a color filter 23 for achieving colors are formed on a second substrate 20. Then, an overcoat film (not shown) for planarizing the color filter 23 can be coated thereon, and alignment films 12a and 12b for determining an initial alignment direction of liquid crystals are coated on the facing surfaces of the first and second substrates 10 and 20. In addition, a liquid crystal layer 30 for controlling light transmittance by a voltage applied to the common electrode 6 and the pixel electrode 7 is formed between the first and second substrates 10 and 20.
In the IPS mode LCD device having the above-described structure, each pixel is switched by the TFT 9, and a pixel voltage can be maintained by the storage capacitor Cst However, in terms of the structure of the TFT, the gate electrode 1a and the source electrode 2a overlap with each other and a parasitic capacitance is generated at such overlap portions which changes the pixel voltage. In addition, the parasitic capacitance is changed according to an area where the gate electrode and the source electrode overlaps and an area where the gate electrode and the gate electrode overlaps, respectively. In particular, according to the tendency of enlargement in substrate size, a stitch model, which is subjected to an exposure process several times with respect to the same pattern, has a problem that an area of the overlap region (between the gate and source electrodes and between the gate and drain electrodes) is different at each pixel. If the overall area of the TFT is different, the parasitic capacitance is different, so a variation rate of the pixel voltage is also different. Accordingly, picture quality of the LCD device may be degraded due to the non-uniformity of the parasitic capacitance according to a position of the pixel.